1. Field of the Invention
This invention pertains generally to detection and annunciation apparatus and, more particularly, to such apparatus for detecting and annunciating when a cable limiter or a crab limiter section is operable or when it operates and clears a power circuit. The invention also relates to cable limiters including a fusible element.
2. Background Information
Cable limiters and crab limiters electrically connect and protect low voltage underground secondary network power cables, which make up the backbone of a secondary network power distribution system. The cable limiters are designed to prevent long term overheating of the insulation of the power cables due to sustained over current conditions. The loss of a power cable due to the operation of a cable limiter or a crab limiter section affects the overall robustness of the secondary network power distribution system and can reduce its capability in handling double contingency (N−2) events. The term “double contingency,” or a network power distribution system designed to N−2 conditions, is the number of primary feeders that can be taken out of service or lost due to cable faults and still be capable of handling the total available load current. For example, in a three feeder network, a design that is rated “N−2” can lose 2 of the 3 feeders and still handle the load requirements.
Cable-to-cable limiters are complete units that include a cable-to-cable fusible element, a high temperature filler shell and an insulating sleeve.
Crab limiters provide protection for plural power cables at one common junction. Each power cable is electrically connected to its own separate fusible section. The fusible elements of the fusible sections are encased in a high temperature shell, which provides separate arcing chambers for each fusible section.
Replacing a crab limiter fusible section requires testing every crab take-off and every cable limiter to find the blown fusible element. Performing this operation takes time and costs money. The present design of known cable limiters and crab limiters is such that after they have operated, the entire cable limiter must be cut out and a new cable limiter placed into the power circuit. In the case of the crab limiter, after the last fusible element has cleared, the entire crab limiter is discarded and a new crab limiter is installed. For example, a 5-way/5-way crab limiter has five secondary cables coming in one side of the device, for which all five secondary cables have limiters. These are attached to five secondary cables outgoing from the opposite side of the device. Again, all five secondary cables have limiters. When the last limiter section has cleared, the entire crab limiter is removed and discarded.
There is room for improvement in cable limiters.
There is also room for improvement in crab limiters.